1. Technical Field
The present invention generally relates to a device for heating batteries. More particularly, the present invention is directed to an electrical circuit for using the stored energy of a battery and a resonant element to heat the battery or a group of batteries.
2. Discussion
The presently known battery technologies, including lead acid, NiCad, NiMH, or Li-Ion batteries used in electric or hybrid electric vehicles, or in conventional vehicles, need some form of heating at cold, particularly extreme cold temperatures before they become fully capable of supplying the power and energy for which they are designed. Traditional methods for warming starter batteries used in vehicles include leaving the headlights on for a few moments, or even applying a short at the battery terminals momentarily to warm the battery and increase its energy delivering capacity. These methods, including those using an external energy source to warm the battery, tend to waste a substantial amount of energy outside the battery in order to obtain a proportionately small increase in the battery internal temperature.
Accordingly, it is desirable to provide an electric self-heating battery circuit with a high degree of efficiency. The device and technique described herein uses the concept of resonance for creating a bidirectional flow of energy in the battery, back and forth between the positive and negative electrodes of the battery or battery pack. The bidirectional flow of energy is a natural consequence of resonance and results in an internal heating of the battery without wasting energy outside the battery itself. To set up the resonance, two different forms of energy storage are needed, an inductor that stores energy in the magnetic field as the current builds up in the inductor, and a capacitor that stores energy in the electric field as the voltage rises across the capacitor terminals. A switching device such as an IGBT or a MOSFET will be needed to excite the circuit natural frequency of oscillations according to the following criteria: ##EQU1##